A "psychological comedy founded on Darwin's theory of the Genesis of Man," as the film's subtitle puts it, about a scrawny but intelligent caveman called Weak Hands (Robert Harron), whose romance with the beautiful Lily White (Mae Marsh) is threatened by the big, burly Brute Force (Wilfred Lucas). Setting out to win back his lost love, Weak Hands conceives of the first weapon to defeat his opponent, attaching a sharp rock to a stick which he uses as a club to bludgeon Brute Force to death. Brains win out over brawn. The whole "caveman" gimmick is framed as a flashback device, bookended by modern day segments in which a grandfather preaches the importance of non-violence to his grandson after he catches the boy antagonizing his sister with a stick (his message of non-violence would seem to be contradicted by the outcome of his story, but no matter).
As the description indicates, it's very heavy-handed and blunt in its storytelling and characterizations, and contains none of the technical innovations that mark Griffith's most interesting work from this period. It seems that by 1912, he was clearly reaching the limitations of narrative filmmaking within the one- and two-reel format. Griffith was quoted as comparing the production methods at Biograph to "grinding out sausages", a furious rate of production which of course allowed him to learn much and make great strides in a short period of time, but which has its inevitable drawbacks that become clear in an uninspired effort like this. His "pet project" during his last days at Biograph was the four-reel JUDITH OF BETHULIA, which pointed the way to the large-scale epics on which he'd soon embark. MAN'S GENESIS feels like Griffith going through the motions.
The film is available for viewing at the Internet Archive.